Microfold (M) cells: important immunosurveillance posts in the intestinal epithelium

The transcytosis of antigens across the gut epithelium by microfold cells (M cells) is important for the induction of efficient immune responses to some mucosal antigens in Peyer’s patches. Recently, substantial progress has been made in our understanding of the factors that influence the development and function of M cells. This review highlights these important advances, with particular emphasis on: the host genes which control the functional maturation of M cells; how this knowledge has led to the rapid advance in our understanding of M-cell biology in the steady-state and during aging; molecules expressed on M cells which appear to be used as “immunosurveillance” receptors to sample pathogenic microorganisms in the gut; how certain pathogens appear to exploit M cells to infect the host; and finally how this knowledge has been used to specifically target antigens to M cells to attempt to improve the efficacy of mucosal vaccines

Secretory IgA's complex roles in immunity and mucosal homeostasis in the gut.

Secretory IgA (SIgA) serves as the first line of defense in protecting the intestinal epithelium from enteric toxins and pathogenic microorganisms. Through a process known as immune exclusion, SIgA promotes the clearance of antigens and pathogenic microorganisms from the intestinal lumen by blocking their access to epithelial receptors, entrapping them in mucus, and facilitating their removal by peristaltic and mucociliary activities. In addition, SIgA functions in mucosal immunity and intestinal homeostasis through mechanisms that have only recently been revealed. In just the past several years, SIgA has been identified as having the capacity to directly quench bacterial virulence factors, influence composition of the intestinal microbiota by Fab-dependent and Fab-independent mechanisms, promote retro-transport of antigens across the intestinal epithelium to dendritic cell subsets in gut-associated lymphoid tissue, and, finally, to downregulate proinflammatory responses normally associated with the uptake of highly pathogenic bacteria and potentially allergenic antigens. This review summarizes the intrinsic biological activities now associated with SIgA and their relationships with immunity and intestinal homeostasis

Collaboration of epithelial cells with organized mucosal lymphoid tissues

Immune surveillance of mucosal surfaces requires the delivery of intact macromolecules and microorganisms across epithelial barriers to organized mucosal lymphoid tissues. Transport, processing and presentation of foreign antigens, as well as local induction and clonal expansion of antigen-specific effector lymphocytes, involves a close collaboration between organized lymphoid tissues and the specialized follicle-associated epithelium. M cells in the follicle-associated epithelium transport foreign macromolecules and microorganisms to antigen-presenting cells within and under the epithelial barrier. Determination of the earliest cellular interactions that occur in and under the follicle-associated epithelium could greatly facilitate the design of effective mucosal vaccines in the future

The Family Streptomycetaceae

The family Streptomycetaceae comprises the genera Streptomyces, Kitasatospora, and Streptacidiphilus that are very difficult to differentiate both with genotypic and phenotypic characteristics. A separate generic status for Kitasatospora and Streptacidiphilus is questionable. Members of the family can be characterized as non-acid-alcohol-fast actinomycetes that generate most often an extensively branched substrate mycelium that rarely fragments. At maturity, the aerial mycelium forms chains of few to many spores. A large variety of pigments is produced, responsible for the color of the substrate and aerial mycelium. The organisms are chemoorganotrophic with an oxidative type of metabolism and grow within different pH ranges. Streptomyces are notable for their complex developmental cycle and production of bioactive secondary metabolites, producing more than a third of commercially available antibiotics. Antibacterial, antifungal, antiparasitic, and immunosuppressant compounds have been identified as products of Streptomyces secondary metabolism. Streptomyces can be distinguished from other filamentous actinomycetes on the basis of morphological characteristics, in particular by vegetative mycelium, aerial mycelium, and arthrospores. The genus comprises at the time of writing more than 600 species with validated names. 16S rRNA gene sequence-based analysis for species delineation within the Streptomycetaceae is of limited value. The variations within the 16S rRNA genes—even in the variable regions—are too small to resolve problems of species differentiation and to establish a taxonomic structure within the genus. Comprehensive comparative studies including protein-coding gene sequences with higher phylogenetic resolution and genome-based studies are needed to clarify the species delineation within the Streptomycetaceae